The present invention is directed to an improved thermally conductive conformable interface layer designed for use in mounting electronic devices such as transistors or the like onto the surface of a chassis or heat sink. The thermally conductive interface material of the present invention is readily conformable, and forms a highly conductive thermal bridge or transfer medium between surfaces of the electronic device and the external surfaces of the electronic device and the heat sink. The devices of the present invention find particular application in use with solid-state electronic devices such as transistors and the like.
The thermally conductive material of the present invention comprises a multi-layer laminate, typically a polymeric thermally conductive layer along with a film of adhesive bonded thereto. While the polymeric layer is preferably a silicone resin, other materials have been found useful such as, for example, certain polyamides including polyester amide copolymers prepared using isophthalic acid, ethylene diamine, ethanol amine, and a diacid prepared from the dimerization of tall oil acid and piperazine. Other useful materials for the preparation of the polymeric layer include polydimethylsiloxane blended with a thermoplastic adhesive such as a linear saturated polyester.
When a polymer layer comprising polydimethylsiloxane is utilized, it is normally filled with a thermally conductive filler such as graphite, and an adhesive layer is utilized to attach the silicone polymeric layer onto the device or onto the heat sink. For ease in handling and processing, a film or liner is utilized for temporarily mounting and supporting the polymeric silicone/adhesive composite. In addition to their desirable mechanical properties and their particularly low thermal resistance, devices prepared in accordance with the present invention are designed to facilitate ease of application to surfaces of electronic devices (typically semiconductors) and also to the surfaces of heat sinks and/or heat spreaders.
In the electronics industry, there is a constant desire, impetus and need to decrease the size of assemblies and sub-assemblies. This results in a corresponding increase in the circuit density. Simultaneously, there is a constant endeavor present to increase the power handling capability of circuitry including micro-circuitry, thereby resulting in the generation of greater quantities of thermal energy or heat. The quantity of thermal energy being generated requires improved methods for removal and dissipation of the heat so that the thermally sensitive components present in the circuits will not deteriorate or fail. At operating temperatures reaching and/or exceeding 80.degree. C., certain components tend to quickly fail, or alternatively, the performance of such components will rapidly deteriorate. Numerous, if not constant, attempts have been made to increase the rate of heat dissipation through the use of polymeric compositions filled with various thermally conductive fillers. Typical of the patents showing these efforts are U.S. Pat. No. 3,908,040, along with others. The present invention represents a significant improvement in the thermal heat dissipation properties of interface layers used for mounting solid-state electronic devices such as transistors or the like onto heat sinks and/or heat spreaders. The devices of the present invention furthermore represent an improvement in facilitating the application of the thermal interface material to the surfaces of the devices which are required to be mounted on a chassis, heat sink or heat spreader.
In one embodiment of the present invention, the polymeric layer of the interface is bonded to the surface of a metallic chassis utilizing a heat-seal or thermally activated adhesive. It is recognized that certain silicone-based polymers such as, for example, some polydimethylsiloxanes, polydiphenylsiloxanes, and/or other siloxane polymers containing siloxane repeating units may be inherently difficult to achieve bonding. One feature of the present invention includes a method for bonding thermoplastic adhesives onto the silicone-based polymeric layer.
Reduction in thermal resistance through the interface may be achieved by utilizing thermally conductive fillers, such as, for example, graphite, alumina, boron nitride, titanium boride, and/or mixtures of these materials.
The interface devices of the present invention are particularly adaptable for use with a conventional hot stamp application machines, including those which are typically used in industry to apply labels to various plastic and/or metal surfaces.